Trailing backslash at end of expression: `` when searching for valid regexp

Created by: slimsag

https://sourcegraph.com/search?q=repo:%5Egithub%5C.com/sourcegraph/sourcegraph%24+%28%5C+%7C%5C.%7C%29getOrganization%5C%28&patternType=regexp

This is a perfectly valid re2 expression: https://play.golang.org/p/qxzlrpx1ecm

The problem is by our query parser as two seperate regexp terms: (\ |\.|)getOrganization\( becomes (\ and |\.|)getOrganization\( since we split using spaces.

AFAIK we have some planned work already to make this better in the near future I believe, filing this for tracking purposes anyway.